Some black hole systems display a relativistically broadened
iron emission line in their spectrum, presumably due to
fluorescence in the surface layers of the inner regions of
the accretion disk. Constellation-X will allow us to observe
variability in this line due to the orbital motions of
inhomogenieties in the disk as well as reverberation effects
(i.e., the light echo of short/intense flares across the
accretion disk). We will show how these phenomena can be
used as a quantitative probe of black hole mass, spin and
(possibly) the validity of General Relativity.